Mongodb – Mongoose returning empty array after populate() method

Answering my own question because I struggled with this a lot and want to help future people reading this question. First of all and really important issue might be if you are not specificying a collection name. In order to specify a collection name add a third argument in your model creation like so:

export default mongoose.model<OrderLineDocument>('OrderLine',orderLineSchema,'orderLines')

Before the third argument mongoose was creating a collection name with a lower-cased plurarized model name - orderlines. But when I was trying to populate my response I was referring to orderLines with a capital L. So heads up always specify your collection name or check your collection name in your MongoDB Shell/ MongoDB Atlas.

Second of all. Always check if the _ids your array/object that you want to populate are correct _ids that exist in your database. If they are non-existent or wrong the response would either contain empty array or even would return error.

For example I was adding this in the beginning when the error was occuring but those _ids were not existing (I had deleted them before but forgot) in my orderLines collection in the first place (you can still add _ids that are not existing if they are the correct _id structure so be aware)

POST http://localhost:5000/api/v1/orders/62326179b9c85d3fc833d686
Content-Type: application/json

{
  "orderedlines":[
      {"_id": "6251be4d4186c34788e4f034"},
      {"_id": "6251be494186c34788e4f030"}
  ],
  "totalPrice":"121.99"
}

Third of all. If you are using TypeScript there are three approaches that work:

1. Using your own type

export type OrderLine = {
  _id: string
}

export type OrderDocument = Document & {
  userId: string
  orderedlines: OrderLine[],
  totalPrice: number
}

2. Saying this would be an array of string

export type OrderDocument = Document & {
  userId: string
  orderedlines: string[],
  totalPrice: number
}

3. Array of ObjectIds

export type OrderDocument = Document & {
  userId: string
  orderedlines: mongoose.Schema.Types.ObjectId[]
  totalPrice: number
}

After all of the above three things are taken in consideration what worked for me in the end was this approach:

const findOrdersForUserId = async (
  userId: string
): Promise<OrderDocument[]> => {
  const ordersToReturn = await Order.find({ userId: userId })
    .sort({ _id: 1 })
    .populate('userId')
    .populate({
      path: 'orderedlines',
      populate: { path: 'productId', model: 'Product' },
    })
  return ordersToReturn
}

And this is a part of the response I am getting just to showcase that both orderedline, productId and userId are being populated correctly

{
        "quantity": 4, //part of a single orderedline object
        "_id": "6251be494186c34788e4f030", //part of a single orderedline object
        "productId": { // beginning of populated productId
          "_id": "623b3b24da0e7828602c78e3",
          "name": "AMY SS14R - TRICKSTER",
          "image": "https://res.cloudinary.com/dtggdx3hc/image/upload/v1648048425/amy-deluxe-stick-steel-r-ss09r10_1000x1000_bfngtl.jpg",
          "price": 1299,
          "__v": 0
        }, // end of populated productId
        "price": 2.99, //part of a single orderedline object
        "__v": 0 //part of a single orderedline object
      }
    ], // end of the orderedline array of objects
    "_id": "6252bfa8dd00081d7c273e4d", // _id of the order
    "userId": { //beginning of populated userId
      "favourites": [
        "623b39e684b9baf1109053f8",
        "623b3afada0e7828602c78df",
        "623b3b49da0e7828602c78e7",
        "623b39ba84b9baf1109053f7",
        "623b3b59da0e7828602c78e9"
      ],